1. Field of the Invention
The present invention relates to a robot calibration system with a linear displacement measuring device.
2. Description of the Related Art
There are many known calibration systems for improving the positional accuracy of an industrial robot which are based upon a kinematic model of the robot. The movement of a single robot is controlled by algorithms executed within the processor for the robot (the robot""s xe2x80x9ccontrollerxe2x80x9d). These algorithms are based upon a mathematical model of the robot""s geometry based on ideal, nominal parameters (ie. length of each link, twist angles between links, etc). However, the actual parameters (also known as xe2x80x9cas builtxe2x80x9d parameters) of an individual industrial robot differ from the nominal ones due mainly to tolerances applied to each component in both the machining of components, sub-assembly of components, and final assembly of an industrial robot. Consequently, each individual robot of the same production model generally possesses a set of actual/xe2x80x9cas builtxe2x80x9d parameters.
Therefore, a loss of xe2x80x9cabsolute robot positional accuracyxe2x80x9d results, for example, when programming an individual industrial robot xe2x80x9coff-linexe2x80x9d (ie. programming by indicating Cartesian coordinates for a desired robot position rather tan driving the robot to that desired position), due to use of the nominal parameters of the robot (instead of the xe2x80x9cas builtxe2x80x9d parameters) by the robot controller: the robot does not actually achieve the commanded Cartesian coordinates of position in space desired by the robot operator/programmer.
The process of identifying the set of actual/xe2x80x9cas builtxe2x80x9d parameters associated with an individual industrial robot is often referred to as xe2x80x9crobot calibrationxe2x80x9d. There are a number of different known methods which then use these actual/xe2x80x9cas builtxe2x80x9d parameters, versus the nominal ones, to modify the positions in a prouction robot program to improve the robot""s positional accuracy.
More specifically, due to the improved positional accuracy, robot calibration techniques permit the following operations to be performed without modification of robot positions by the robot operator/programmer (a process referred to as xe2x80x9ctouch upxe2x80x9d): (1) programming of the robot xe2x80x9coff-linexe2x80x9d using a PC or workstation-based simulation software product; (2) restoring production robot programs following a collision between a robot and another entity (after regarding-calibration of the robot); and (3) transferring robot programs from one robot to another (ie. compensating for xe2x80x9cas builtxe2x80x9d parameters of each robot which have been identified in the process of calibrating each robot).
The prior art systems for the calibration of a robot generally accomplish their functions by means of executing calibration robot programs on the robot controller which instruct the robot to move through a series of positions in its operational space while being monitored by a measurement device which is capable of determining the three dimensional location (ie. x, y, z location in a particular Cartesian coordinate system) of a point (often referred to as the Tool Center Point or TCP) of the end effector of the subject robot. In some cases, the measurement system provides more or less degrees-of-freedom but typically such measurement systems report measurement data in some type of xe2x80x9cCartesianxe2x80x9d (or linear) format (e.g. 2-dimensional, 3-dimensional, or 6-dimensional).
Among all prior art systems, the purpose of the calibration procedure is to collect information concerning deviation between the actual (as identified by the measurement system) robot position achieved at each position in the calibration robot program and the corresponding commanded robot positions and then use that information to xe2x80x9cdeducexe2x80x9d or calculate the actual/xe2x80x9cas builtxe2x80x9d parameters (ie. the differences between the xe2x80x9cactualxe2x80x9d robot and the xe2x80x9cnominalxe2x80x9d robot parameters). Typically the prior art systems used 3-dimensional or 6-dimensional measurement systems that include, for example, theodolites, laser interferometers, and camera/photogranmmetry systems.
In one known prior art system (the RoboTrack System distributed by Robot Simulations Ltd.) three measurement cables are secured to the end of the robot arm. The other end of each of the measurement cables is connected to a linear displacement measurement device which measures the extension and retraction of the cable due to the movement of the end of the robot arm. The linear displacement measurement devices are positioned at various known locations around the robot""s operational envelope. Once the measurement cables have been connected to the robot, the displacement devices at each robot position measure the distances between the position achieved by the robot arm and the displacement devices. Using triangulation and other mathematical algorithms, the 3-dimensional position (x, y, z in a single Cartesian coordinate position) of the end of the robot arm and the end effector can be determined based upon the linear displacement data which is gathered from each of the measurement devices. This prior art system has numerous problems and in fact is generally only found in non-commercial facilities. Furthermore, this prior art system depends upon the accuracy of the 3-dimensional positional information, which means by nature of the triangulation process that the positional information xe2x80x9cdegradesxe2x80x9d in several portions of the robot""s operational envelope (particularly at the boundaries of such operational envelope). Therefore, in some instances, in fact, the absolute positional accuracy of the robot was not improved but rather worse than before the calibration procedure was performed with this prior art system.
Moreover, in addition to the restrictions upon overall accuracy of this prior art system attributable to triangulation and use of this xe2x80x9cderivedxe2x80x9d 3-dimensional data, the linear displacement measurement devices themselves restricted measurement accuracy due to inherent design flaws. For example, each measurement cable of this prior art system exits the housing of the linear displacement measurement device at various angles/attitudes through a hole. By definition, as the cable can simply not bend at a xe2x80x9csharpxe2x80x9d angle, the xe2x80x9croundingxe2x80x9d of the cable when making contact with the edge of the exit hole contributed to error in the measurement data. Furthermore, this prior art system does not contain a design element to defeat overlap of the measurement cable as it retracts into the housing. This design issue concerning overlap contributes significantly to overall system error as the length of the cable extended is calculated based upon the assumed known and constant radius of the drum upon which the measurement cable retracts.
Other prior art systems have tried to overcome the overlap issue by employing a groove on the drum to force the cable to wind sequentially on the drum. However, as this groove method requires xe2x80x9cspacingxe2x80x9d on the drum surface, the groove method naturally restricts the amount of measurement cable which can be held by each linear displacement device, and consequently restricts the amount of the robot""s operational envelope in which measurements can be recorded. Finally, the groove method does not prevent cases in which the cable xe2x80x9cjumpsxe2x80x9d out of one groove and rests on top of another portion of the drum at unpredictable intervals.
One prior art system avoids any cable issue entirely and the cable itself by employing a radial-distance linear transducer referred to in the art as (an LVDT or xe2x80x9cball-barxe2x80x9d) instead, that is often referred to as the telescopic ball-bar system. The ball-bar mechanism of this prior art system has a magnetic chuck permanently mounted at one end, and a removable high precision steel ball mounted at the opposite end. Extension bars permit the nominal length of the ball-bar to be increased in order to reach more of the robot""s operational envelope, but these extension bars add significant weight (and corresponding force) at the measurement point and therefore degrade the accuracy of the measurement data recorded with the LVDT mechanism.
The inventors of this prior art system state that ideally this prior art system would require use of six ball-bars in order to completely identify the robot endpoint pose at every posture. However, this prior art system alternatively permits, although the process is cumbersome, the operator to xe2x80x9cserializexe2x80x9d the procedure by commanding the robot to travel on a spherical shell while only one ball-bar is connected between its end point and table. This alternate procedure must be repeated six times while interchanging the connections between the three balls and three magnetic chucks in six appropriate combinations. In this way, this prior art system allows collection of the necessary measurement data with a single ball-bar at the cost of extra time. Such extra time is a premium price to pay in the kind of production environment in which robot systems are typically deployed.
In fact, the inventors of this prior art system state that the limited reach of the ball-bar substantially restricts the positional freedom that can be achieved during the calibration process. This restriction upon the size of the measurement envelope of this prior art system is the basis for the requirement that the operator mount at least three (preferable six) magnetic chucks within the robot""s operational envelope in order to record robot position measurements in as large an area as possible. Unfortunately, the requirement that a plurality of magnetic chucks be employed prevents use of this prior art system to perform robot calibration automatically (ie. without robot operator/programmer intervention).
This prior art system developed utilizes a rotatable drum about which the measurement cable is coiled. As the cable extends and retracts from the measurement device, the drum rotates. An optical encoder measures the rotational movement of the drum in order to determine the linear displacement of the end of the cable. The cable is coiled a plurality of times around the drum, extends at least partially around a first pulley, at least partially around a second pulley positioned adjacent to the drum and extends out of the housing of the device to a first end which is secured to the end of the robot arm or robot end effector. These pulleys have a known radius, and eliminate the problem of the cable exiting through a hole at different angles, although they add some inherent complexity in the measurement process, since the measurement cable length no longer represents a straight line from one point to the other. Knowing the circumference and rotational displacement of the drum, the linear displacement of the measurement cable can be calculated. As indicated above, in order to insure accuracy, it is imperative that the cable be wound in a single layer on the outer surface of the drum (ie. no xe2x80x9coverlapxe2x80x9d). If the cable overlaps itself on the drum, the effective outer circumference about which the cable is coiled will be increased. The result of this situation would be a reduced rotational displacement about the drum when the cable is either extended or retracted, thereby providing inaccurate information concerning measurement cable displacement.
The design element employed by this prior art system of the present inventor consists of spacing the first pulley a sufficient distance from the drum (that distance being a function of the cable thickness and the number of coils about the drum). This distance must be sufficient such that the angle at which the cable comes on the drum is shallow enough (so that it is nearly always perpendicular to the drum)xe2x80x94to insure that, as the cable is wound onto the drum, the thickness of the cable itself prevents the cable from overlapping. However, this increase the size of the displacement measurement device itself, proportionally to the amount of coils around the drum.
Several years ago the inventor of the present invention developed the following prior art system which included design elements which resolve these measurement cable issues. Two such linear displacement measurement devices were located at a fixed, known distance one relative to the other on a single mounting surface approximately 1500 mounting member in length. Although one end of each of the two measurement cables is coiled on a drum, the other ends of the cables are free to extend in 3-dimensional space. Using the known, constant distance between the two linear displacement measurement devices, this prior art system converts the two linear measurements into a 2-dimensional position (ie. an x, y position in a single Cartesian coordinate system) using triangulation. As a result, this prior art system exhibits some xe2x80x9cdegradationxe2x80x9d of the converted measurement data, similar to the previously discussed prior art system which employs three linear displacement devices to report 3-dimensional position information.
It is an object of the present invention to provide a calibration system which accurately identifies robot, end-effector, and fixture parameters using 1-dimensional multi-directional data directly and does not require conversion to Cartesian (e.g. 2-dimensional xy data or 3-dimensional xyz data). It is a further object of the present invention to provide a linear displacement device offering high accuracy measurements by preventing cable overlap with a compact design, and a large measurement volume by increasing the amount of measurement cable which the linear displacement device can accommodate with minimal increase in the overall size of the linear displacement device itself.
While the foregoing drawings, description, and discussion show some specific embodiments in the invention, yet other variations thereof will be apparent to one of sill in the art. For example, the cable assembly which is used to generate the one dimensioned position signal may be replaced by any other system which can generate a signal in response to linear displacement. Such other systems include optical, mehcanical, or electronic encoders, and the one-dimensional position signal can be processed in accordance with the method of this invention to measure displacement of the robot end point.
The present invention provides an improved device for calibration of a robot system including a linear displacement measurement device that accurately identifies robot, end-effector, and fixture parameters using 1-dimensional multi-directional data directly. In the primary embodiment, the linear displacement measurement device comprises a housing, an elongated member, a drum, a shaft, a rotation sensor, a means for moving the drum axially with respect to the shaft as the elongated member is wound about the drum, a set pulleys to guide the elongated member and a system for determining the distance traveled by the elongated member. The shaft is rotatably mounted in the housing. This arrangement ensures that the elongate member is wound about the drum in a single layer without overlapping itself. The shaft (on which the drum is mounted) rotates together with the drum, allowing mounting of a rotation sensor to the shaft. This sensor thus measures the true rotation of the drum. The information derived from this system determines the linear displacement of the elongated member. The linear displacement measurement information provided by the linear displacement measurement device is used in conjunction with the calibration system software to perform calibration of a robot system.
Unlike the prior art systems, the present invention improves the accuracy of the robot parameters identified in the calibration process, reduces the number of linear displacement measurement devices required to gather data, allows calibration based on one dimensional data, eliminates variability of data which is present in non-pulley devices and reduces the size of the displacement measurement device.